When I pulled the sending unit out of my car, it didn't even have that piece on it. Also my float was half disintegrated. Anything submerged in gasoline for 50 years is going to be in rough shape. Classic Industries shows a part No. K404 filter for a 5/16" pickup tube. It looks like it may fit over the piece you have. So I wouldn't get rid of that collar-piece just yet.
